1. Field of the Invention
The present invention relates to preventing a substrate from being deformed due to thermal stress or deposition stress.
2. Description of the Related Art
Recent mobile display devices and flexible display devices, which are increasingly demanded, are required to be thin, light, and unbreakable.
Mobile display devices and flexible display devices can be thin and light by using a thin glass substrate. This thin glass substrate cannot be easily handled and accurately aligned and is weak with respect to withstanding external impact. To solve these problems, a method of using an existing glass substrate and then chemically or mechanically thinning the glass substrate has been adopted. This method, however, is complicated, and the thinned glass substrate is still weak with respect to withstanding external impact. Thus, this method cannot be used in practice.
To solve these problems resulting from the use of glass substrates, a method of using a plastic substrate that is more flexible than the glass substrates and not easily damaged by external impact has been used. However, this plastic substrate has a low thermal resistance, so it cannot bear a high-temperature process, such as, a formation of a polycrystalline silicon thin film transistor and other depositions.
To solve these problems, a method of using a metal substrate having high thermal resistance and high flexibility has been proposed. However, when a layer is deposited on a metal substrate and the resultant metal substrate undergoes a high-temperature process, the resultant metal substrate is deformed due to a difference between thermal expansion coefficients of the substrate and the layer. For example, when the thermal expansion coefficient of the layer is greater than that of the metal substrate, the metal substrate is convex in a direction of the layer. To the contrary, when the thermal expansion coefficient of the metal substrate is greater than that of the layer, the metal substrate is convex in the direction opposite to the direction of the layer. Of course, this problem can occur not only using a metal substrate but can also occur using other substrates.